Ultrasound imaging is a technique for non-invasively creating an image of the inside of a test subject including a human body, through the use of ultrasound wave (sound wave not intended for hearing, and generally high-frequency sound wave having 20 kHz or higher). By way of example, a medical ultrasound imaging apparatus will be briefly explained. An ultrasound probe transmits the ultrasound waves to the inside of a patient, and receives echo signals reflected from the inside of the patient.
The received signals are subjected to signal processing in one or both of the ultrasound probe and the main unit of the ultrasound imaging apparatus, and thereafter transferred to a monitor and an ultrasound image is displayed thereon. More specifically, for example, a transmit beamformer in the main unit of the ultrasound imaging apparatus generates signals of a transmission beam, allowing the signals to pass through the transmit-receive separation circuit, and thereafter transfers the signals to the ultrasound probe. The ultrasound probe sends out the ultrasound waves. After receiving echo signals from the internal body, the ultrasound probe transmits the signals to the main unit of the imaging apparatus. In the main unit of the imaging apparatus, the received signals pass through the transmit-receive separation circuit and the receive beamformer, and those signals are transmitted to an image processor. The image processor executes various imaging processes using various filters, a scan converter, and the like. Finally, the monitor displays an ultrasound image.
As described above, a general ultrasound diagnostic apparatus is made up of three techniques; transmit beamforming, receive beamforming, and a backend imaging processing. Particularly, since the beamformers for transmitting and receiving perform signal processing at an RF (high-frequency) level, algorithms and implementation architecture in the beamformers decide a basic image quality of the ultrasound image. Therefore, the beamformers serve as major parts of the apparatus.
The receive beamformer assigns a delay time to each received signal (received data) in multiple elements that constitute the ultrasound probe, the delay time distributing an amount of delay in a concave form, in association with the relations between a focal position and the element positions, and after virtually obtaining the focal point (focused) at a certain point in space, the received signal data items are summed up. This method is referred to as a beamforming according to a delay-and-sum method. In this delay-and-sum method, the received data items that are received by the multiple elements in the ultrasound diagnostic apparatus are multiplied by a fixed weight vector stored in the diagnostic apparatus, and the delay is implemented according to this processing means. This process is also performed in the transmit beamformer in a similar manner, not only in the receive beamformer.
On the other hand, as a basic problem of the ultrasound imaging apparatus, it is known that lateral resolution is subject to constraints. Since transmitting and receiving of the ultrasound waves are performed by an array having a finite opening size, there is an impact of diffraction at the edge of the opening. If an infinitely long array is prepared, there is a possibility that the resolution is enhanced infinitely in the same manner as in the depth direction. In actual, however, a physical restriction in designing the apparatus, i.e., the length of the array, has hampered the enhancement of the lateral resolution. In recent years, it is attempted that the aforementioned fixed weight vector used for delaying, upon summing of delays by the beamformer, is changed adaptively for the time-series transmit-receive data items, one by one, thereby obtaining an ultrasound image of higher definition, and this attempt is coming to attention. Accordingly, there is a possibility that this brings a marked improvement in the lateral resolution, being one of essential problems in the beamforming technique.
Particularly in recent years, the patent document 1, for example, discloses that an adaptive signal processing technique including the MVDR method (Minimum Variance Distortionless Response; Capon method) that has been developed in the field of mobile communication is applied to the ultrasound imaging process. By using the adaptive method, the weight vector being a fixed value conventionally, is obtained for each sample point of the received signal in the time direction, and the received signal is multiplied by this weight vector, thereby achieving delay.